1. The Keyboard Controller Essential steps for implementing an interrupt service routine for the Keyboard Controller

2. Our near-term goal To build a ‘mini’ x86 operating system It should be able run a Linux application It needs to support: –basic console input/output (keyboard, screen) –handling of device-interrupts (timer, keyboard) –‘exit’ gracefully when application is finished –utilize x86 privilege restrictions and 32bit code Most needed building-blocks already done

3. Keyboard and its controller Keyboard Controller CPU PIC bus KEYBOARDPS/2 MOUSE output port TIMER output bufferstatus regcontrol reg 0x64 0x60 input buffer input port RAM PIC

4. 6 KB Controller Status PARETIMAUXBKEYLC / DSYSFINPBOUTB 7543210 LEGEND OUTB: Output-Buffer Full (1=yes, 0=no) INPB: Input-Buffer Full (1=yes, 0=no) SYSF: System-Flag (1=self-test successful, 0=power-on reset) C/D: Command/Data was written to (1=port 0x64, 0=port 0x60) KEYL: Keyboard-Lock status (1=keyboard available, 0=locked) AUXB: Output-Buffer’s data is (1=for mouse, 0=for keyboard) TIM: General Timeout-Error has occurred (1=yes, 0=no) PARE: Parity-Error on last byte from keyboard/mouse (1=yes, 0=no)

5. A few Controller Commands 0xAD: Disable Keyboard 0xAE: Enable Keyboard 0xA7: Disable PS/2 Mouse 0xA8: Enable PS/2 Mouse 0xC0: Read input-port (to output buffer) 0XD0: Read output-port (to output buffer)

6. Interrupt-Handler actions When the keyboard-controller issues an interrupt, these actions should be taken: –Save values in the working CPU registers –Read keyboard-controller’s status-register –If the output-buffer has new data, read it –If a “special” key (shift/toggle), adjust kb_flags –For a normal key, translate scancode to ascii –Insert the code-pair at tail-end of kb-queue –Send EOI-command to Interrupt Controller –Restore the saved values from CPU registers –Resume the interrupted procedure (with ‘iret’)

7. Keyboard Queue KBHEAD KBTAIL

8. BIOS DATA AREA KBFLAGS is word at address 0x40:0x17 KBHEAD is word at address 0x40:0x1A KBTAIL is word at address 0x40:0x1C KBQUEUE is array of 16 words array base-address at 0x40:0x1E KBBASE is word at 0x40:0x80 KBTAIL is word at 0x40:0x82

9. Format of KBFLAGS LSB Left-Shift key is pressed (1=yes, 0=no) Right-Shift key is pressed (1=yes, 0=no) 01 Ctrl-Key is pressed (1=yes, 0=no) Alt-Key is pressed (1=yes, 0=no) 234567 Caps-Lock is active (1=yes, 0=no) Num-Lock is active (1=yes, 0=no) Scroll-Lock is active (1=yes, 0=no) Insert-mode is active (1=yes, 0=no)

10. Demo ‘minikybd.s’ It shows a minimal implementation for the keyboard-controller’s interrupt-handler It runs in Real-Mode It ‘echos’ a user’s keystrokes IN-CLASS EXERCISE: modify this demo to run in Protected-Mode